1. Field of the Invention
The present invention pertains to the field of fiber-reinforced thermoplastic composites. More particularly, the invention pertains to an improved process for preparing such composite structures wherein the time required to fully consolidate the composite is reduced significantly, and to the products produced thereby. This improvement is achieved through the use of commingled yarn containing reinforcing fibers and thermoplastic matrix fibers which have been sized with an emulsion or dispersion of a thermoplastic which is melt-compatible with the thermoplastic fibers but which has a melting point significantly lower than the thermoplastic matrix fibers.
2. Description of the Related Art
Thermoplastic matrix fiber-reinforced composites are now well known items of commerce. Such products may be prepared by numerous methods. Glass mat reinforced polypropylene composites, for example, are prepared by impregnating needled or non-needled continuous or chopped fiber layers with molten polypropylene to form an intermediate, somewhat lofty product, which may then be consolidated under heat and pressure to form composite structures having a thickness considerably lower than the intermediate product. Such products are particularly useful in the transportation industry for body panels and engine covers. Complex shapes are difficult to achieve with this process.
A similar process is resin transfer molding. By this process, a plurality of fiber-reinforced laminae interspersed with sheets of thermoplastic or having thermoplastic sheets exterior to one or both sides of the laminate layup are consolidated under heat and pressure whereupon the thermoplastic flows into the fiber reinforcing laminae. Such products often do not have the same physical properties as thermoplastic composites formed from thermoplastic impregnated fiber-reinforced prepregs.
The use of thermoplastic impregnated fiber reinforced prepregs is now commonplace. Such prepregs are stacked together and consolidated under heat and pressure. However, because the prepregs themselves are stiff and "boardy", it is difficult and in many cases impossible to prepare composites which are other than planar or modestly curvilinear.
Improvements in forming thermoplastic composites of complex shapes are illustrated by the use of thermoplastic powder impregnated prepregs and the use of hybrid or commingled yarns. In the former case, unidirectional or woven fiber reinforcement is impregnated with the necessary quantity of thermoplastic in the form of a fine powder. A binder, generally a high molecular weight polyoxyethylene glycol, polyvinylalcohol, polyacrylate, polyvinylpyrollidone or similar water or alcohol soluble polymer is usually necessary to provide integrity, particularly with unidirectional fibers, and to prevent loss of the thermoplastic powder from the prepreg during shipping and handling. The presence of the binder can result in an undesirable decrease in composite physical properties. Such powder impregnated prepregs, however, can be utilized to prepare composites of complex shapes due to their drapeability, particularly when wet.
Hybrid and commingled yarns have also proven successful in preparing composites of complex shapes. In the remainder of the specification, the term "hybrid yarn" will be used to define yarns which are combinations of reinforcing fibers and thermoplastic matrix fibers, whether truly commingled, i.e. having relatively statistical or uniform distribution of the different fiber types; or whether served, core/shell, partially commingled, or co-wound. Of course, commingled yarns are the preferred hybrid yarns. The term "thermoplastic matrix fibers" refers to the hybrid yarn fibers which will fuse during consolidation to form the thermoplastic matrix to distinguish these thermoplastic fibers from thermoplastic fibers which may serve as reinforcing fibers, and which do not fuse during consolidation.
Commingled yarns may be prepared as disclosed in U.S. Pat. Nos. 4,799,985 and 4,818,318 which are herein incorporated by reference. In general, fiber reinforcing tows are spread to form a unidirectional tape while thermoplastic tows are similarly spread and superimposed upon the fiber tape generally under low or relatively modest pressure. Combs or air jets are then utilized to more thoroughly intermix the reinforcing and thermoplastic fibers whereupon the commingled tape is then reformed into a commingled tow or yarn. In some cases, a slight twist is imparted to the yarn to aid in maintaining its integrity. The commingling produced by this process results in a product having a uniform distribution of reinforcing fibers and thermoplastic fibers. This uniform distribution is important, as the physical properties of consolidated structures decrease when non-uniform fiber distribution occurs, a condition aggravated as the melt viscosity of the thermoplastic increases. While cowound yarns (essentially not commingled) have been used with low melt viscosity thermoplastics such as the aliphatic nylons, even here the commingled yarns produce composites with superior properties when compared to cowound yarns of the same composition.
In order to weave or braid hybrid and commingled yarns, a certain degree of yarn integrity is necessary. Limited success has been achieved by twisting the yarn, for example at 0.2-0.8 twists per centimeter. However this level of twist is not totally satisfactory since it tends to damage the delicate reinforcing fiber filaments. Moreover, the composite properties are reduced due to the twist. To assist in fiber integrity, sizes may be applied. However use of traditional sizes such as polyoxyalkylene polyethers, polyvinylpyrollidone, polyvinylalcohol, polyvinylacetate, polyacrylates and the like, while capable of preparing tow with improved weaving and braiding characteristics, again results in a decrease in physical properties which may be severe at times depending on the chemical makeup of the size. This decrease in physical properties becomes more severe as the level of size increases. However, even when such sizes are applied at practical levels, moderate twist must often be applied to further assist yarn integrity.
The commingled yarns produced as described above may be woven into fabric "prepregs" or braided into a variety of braided "prepregs" including ribbons, ropes, and hollow tubular braids. In the preparation of composite products, one or more of such prepregs are laid up together and consolidated under heat and pressure. The time required for full consolidation varies with the melt viscosity of the thermoplastic, the degree of commingling, and the pressure applied. Lengthy consolidation times are commonplace, especially with non-planar structures which do not lend themselves to application of high consolidation pressures. These long consolidation times increase the cost of the finished products.
It would be desirable to devise a process whereby the twist imparted to commingled yarns could be reduced significantly or avoided entirely, and where any size applied would not significantly lower physical properties of the finished composite. It would further be desirable to lower the consolidation time and/or pressure in order to produce finished products in a shorter time and/or at less expense.